Silicon nanostructures including porous silicon (p-Si),1 silicon rich oxides (SROs),2 and freestanding Si nanoparticles have been the focus of intense research because of their unique chemical and optical characteristics. The electronic structure of bulk silicon provides an indirect bandgap of 1.12 eV with the lowest point of the conduction band and the highest point in the valence band occurring at different coordinates in reciprocal space. These restraints make the bandgap optical transition dipole-forbidden, limiting practical optoelectronic application of the bulk crystal due to the low photoluminescence intensity and slow carrier dynamics (i.e., long lived excited states). As the dimensions of a semiconductor particle decrease into the “nano” size regime, the bandgap energy increases and pseudo-continuous bands become discrete energy levels that are populated according to quantum mechanical selection rules. When the particle dimension nears the Bohr exciton radius (ca. 5 nm for silicon), quantum confinement effects emerge and photoluminescence (PL) shifts into the visible spectrum and becomes more intense. Some researchers suggest that the PL observed from photoexcited Si nanoparticles arises because the bandgap transition becomes weakly dipole allowed in this size regime.3 Others claim that the photoemission originates from the passivation of surface traps present in bulk Si.4 Regardless of the explanation, a characteristic photoemission maximum at approximately 1.7 eV is seen for Si-based nanostructures; including the “Si quantum wires” reported by Canham et al.,5 nanocrystalline nc-Si/SiO2 composites,2 and freestanding Si nanoparticles prepared via solution,6,7,8 precursor pyrolysis,9,10 and physical techniques.11,12,13 The unique optical properties and electrochemical stability14 of nanoscale elemental Si offer significant potential for a variety of light emission applications. Furthermore, the biocompatibility of Si and SiO2 makes these materials potentially useful in sensing applications where toxic, electrochemically active compound semiconductor nanoparticles are impractical.
Measuring the direct effect of Si nanocrystal size on the PL spectrum of Si/SiO2 nanocomposites and freestanding Si nanocrystals9,10,13 appears complicated, with interface effects and particle interactions15 playing key roles. Consequently, the size effects and the influence of the indirect bandgap of bulk Si on PL behavior of Si nanocrystals remain poorly understood. Methods for relating PL energy maximum to particle size are diverse and a variety of models have been proposed including: the effective mass approximation,16 empirical tight binding band theory,17 ,18 empirical pseudopotential approximation,19 ,20 and ab-initio local density approximation.21,22 To facilitate better understanding of Si nanoparticle optical and chemical response, straightforward, cost-effective, scaleable methods for preparing materials of controlled size, crystal structure, and surface chemistry are necessary. Well-established physical techniques for preparing Si nanostructures such as porous silicon (p-Si) and silicon rich oxides (SRO) often employ highly corrosive reagents (e.g., hydrofluoric acid), costly procedures (e.g., ion implantation,23,24 vacuum evaporation,25 sputtering,26 and laser ablation27) and provide only partial tailoring of film chemical composition. Furthermore, many of these methods are not easily scalable and are impractical for preparing macroscopic quantities (ca.>500 mg) of material.
SROs are a promising class of nanostructured materials made up of luminescent, crystalline Si nanoparticles embedded in environmentally inert SiO2-like matrices. A common method for preparing SROs employs a multi-step process; the first stage involves deposition of thin “SiO” precursor films using physical methods such as vapor deposition, physical sputtering, or e-beam evaporation to deposit films onto flat substrates.2 These recipe-based approaches control the Si:O ratio by maintaining a specific oxygen flow rate (i.e., partial pressure) during reactive deposition of “SiO” or by controlling the co-deposition rates of Si, “SiO”, and SiO2 to produce SiOx films (0≦x≦2).28 Films are subsequently annealed at high temperature in a reducing atmosphere (typically 4% H2, 96% inert gas) to promote formation of Si nanocrystals.29 Iterative variation of experimental parameters and post deposition micro-probe analyses have previously shown that films with composition ratios close to Si1.0O1.5 that are annealed at ca. 1100° C. produce the strongest PL. Still, fundamental questions remain regarding the mechanism for nanoparticle formation, the relationship between Si particle size and peak PL energy, among others.2 Unfortunately, the exact composition, structure, and purity of “SiO” is subject of a longstanding controversy30,31,32 and is strongly dependent upon processing conditions.30 Further, the exact chemical structure of “SiOx” remains largely ill-defined.33 These uncertainties potentially hinder rational study of chemical composition and its influence on the material properties of SRO nanoparticle composites. Other practical limitations of the abovementioned physical “SiOx” deposition techniques are nontrivial control of chemical composition (e.g., straightforward introduction of dopants), and conformal coverage of textured and “non-flat” substrates (e.g., fiber optic cables) is somewhat limited by these line-of-sight techniques.
Although SROs can be used to provide insight into the photonic, electronic, and chemical interactions of nc-Si within solid matrices, it is essential that these same properties be studied and understood for freestanding Si nanoparticles. This is particularly true if research is to move beyond fundamental investigation and efficient applications are to be realized. Studying the chemical reactivity and tunability of freestanding Si nanoparticles, in concert with single particle spectroscopy could lead to a better understanding and optimization of particle photoemission. To this end effective methods for preparing freestanding Si nanoparticles must be established. While freestanding particles have been dislodged from p-Si surfaces,34 published data suggests individual Si nanoparticles remain trapped in larger (i.e., ≧1 μm) pieces of the p-Si structure.11,12,13 Similar liberation and bulk preparation of Si nanoparticles from SRO matrices is impractical given extremely small sample sizes.35 Laser induced precursor pyrolysis has recently been reported as an efficient method for preparing large quantities of Si nanoparticles from silane at rates of 20-200 mg/hour; this approach however relies on expensive laser equipment and custom designed reactors not available in most synthetic laboratories.9,10 Solution-based procedures,8,36,37,38,39,40,41,42,43 provide some post synthesis material processability but are often plagued by material purity, ill-defined particle surface chemistry, and limited ambient stability—all criteria crucial to the eventual application of these materials in optoelectronic devices.
Silsesquioxanes are commercially available, solution processable, discrete, structurally well-defined molecules composed of silicon-oxygen frameworks with empirical formulae (RSiO1.5) where R may be a variety of chemical functionalities (e.g., H, alkyl, silyl, and aromatic). The chemistry of these compounds is well-established and a variety of cage structures are known.44 Hydrogen silsesquioxane (HSQ), a totally inorganic silsesquioxane (H8Si8O12), is one of the most widely studied and has been investigated as a model silica surface,45,46,47,48 luminescent material,49 and a catalytic support.50,51 Examples of high purity silica have also been prepared from silsesquioxane precursors.52 It is generally accepted that upon oxidative thermal curing, the silsesquioxane cage structure of HSQ collapses to release SiH453 and a SiO2-like network solid forms whose dielectric,54 mechanical, and processing characteristics depend on the curing conditions. Dielectric films produced by thermal curing of HSQ currently find application as spin-on, planarizing dielectric interlayers in the microchip industry.54 To date, no Si nanoparticle preparation employing silsesquioxanes has been reported.
There remains a need for a method for preparing large quantities of Si nanoparticles that, ideally, is uncomplicated, cost-effective and reproducible.